1. Field of the Invention
This invention relates generally to hydrogen storage devices and more particularly relates to such devices utilizing metal hydrides for storage of high volume of hydrogen at lower pressures.
2. Background Art
With the increasing use of hydrogen gas in industrial and commercial applications, conventional methods of storing hydrogen as a gas under pressure are seen to present undesirable risks. Hydrogen storage as a gas is typically done in large, bulky steel cylinders at very high pressures (e.g., 2,000 psi).
Hydrogen may be stored in liquid form, typically in insulated containers at very low temperatures. Energy must be used to keep the temperature low to prevent the liquid hydrogen from evaporating or boiling off. Hence, cryogenic hydrogen production and storage is highly inefficient because of the extremely cold temperatures at which the storage vessels must be maintained.
More recently, attention has been directed to the storage of hydrogen in metallic compounds, known as hydrides. Metal hydrides can store large amounts of hydrogen at low pressures and in relatively small volumes. Low pressure storage of hydrogen in containers containing hydrides is relatively safe and allows the construction of hydrogen containers having forms significantly different than those storing gaseous hydrogen. Examples of low pressure hydride storage containers can be found in commonly assigned U.S. Pat. Nos. 5,250,368, 5,532,074 and 5,623,987, which are incorporated by reference herein.
Additionally, hydrogen storage containers are sold by the assignee of the invention under the trade name Ergenics ST-Series hydrogen storage units utilizing hydrides manufactured by the assignee hereof, for example, HY-STOR® 208 hydride alloys. Additional information about these storage units and hydride alloys is available on the website of the assignee of this invention, found at www.ergenics.com. The use of solid hydridable materials to store hydrogen is also disclosed in numerous patents, such as U.S. Pat. Nos. 3,508,514, 3,516,263, 4,036,944, 5,518,528, 5,697,221 and 5,906,792.
Such metal hydride storage units have many uses and applications in a variety of industrial and laboratory applications and environments. The diversity of applications requires a storage system that can provide the user with a reliable source of hydrogen at a variety of capacities. Small storage units are commonly used as hydrogen sources for laboratory experimentation, as in the examples described above. Much larger units are needed to provide the quantity of hydrogen necessary for vehicular transportation systems or in utility applications. Hydrogen storage systems are needed that will allow for quick and easy use of the hydrogen gas, and the storage system, regardless of the size and capacity of the container, must be robust enough to perform properly and reliably under a wide range of environmental conditions.
Another major consideration is the ability of a user of a hydrogen storage device to measure or gauge the amount of hydrogen gas in the hydride container available for further utilization, as and when needed. An accurate gauge is ideal, but even a gauge of the amount of hydrogen gas availability is desirable that is accurate to within 15–30%.
The construction and normal operation of known metal hydride hydrogen storage systems make it especially difficult to accurately gauge the amount of hydrogen gas available. It is possible to accurately estimate the amount of hydrogen gas available in the above described gas containers which store hydrogen in a gas state from the ideal gas law (PV=nRT), because the amount of hydrogen is directly proportional to the pressure, when temperature is maintained at a constant value. This is possible because in those types of storage systems, the volume, V, and temperature, T, remain essentially constant as an amount of hydrogen gas (n) is utilized and being withdrawn from the system. The utilization of hydrogen gas, as it is withdrawn from the container is indicated by a reduction in the amount of hydrogen (n), and produces a directly proportional reduction in pressure, P. Thus, it is possible to estimate to a very accurate degree, the amount of hydrogen gas available in the container by an accurate reading of the pressure, P.
However, because the hydrogen gas is incorporated into the lattice structure of the metal hydride, storage containers using hydrides cannot rely on the pressure P as a directly proportional measure of the amount of hydrogen gas (n) remaining in the container. It is thus necessary to provide a fairly accurate measure of hydrogen gas contained within a container, similar to a fuel gauge in a standard internal combustion automobile.